Conventionally, there is known a rotary-hearth furnace including an outer circumferential wall, an inner circumferential wall, and an annular rotary hearth interposed between these walls. The rotary hearth, generally, is constituted of an annular furnace body frame, a hearth heat insulator placed on the furnace body frame, and a refractory placed on the hearth heat insulator.
The rotary-hearth furnace having such structure has been used, for example, for heat treating metals such as steel billets or for combustion treating combustible wastes. Recently, a method for producing reduced iron from iron oxide using the rotary-hearth furnace has been focused. An example of such processes for producing reduced iron using the rotary-hearth furnace is described below with reference to FIG. 6 which shows the schematic structure of the rotary-hearth furnace.
Firstly, an iron oxide (such as an iron ore or steelmaking dust) and a carbonaceous reducing material (such as coal or coke) are mixed and granulated to produce a pellet or a briquette (agglomerate). When the pellet or briquette is heated to a temperature area such that a combustible volatile matter to be generated from such pellet or briquette is not ignited, the adhered water thereof is removed to produce a dry pellet or dry briquette.
Such dry pellet or dry briquette (starting material 24 of reduced iron) is supplied to a rotary-hearth furnace 26 using a proper inserting device 23 to form a pellet or briquette layer on a rotary hearth 21. The pellet or briquette layer, while rotating in the black arrow direction, is radiation-heated and reduced due to the combustion of a combustion burner 27 placed on the upper part of the furnace, thereby advancing its metallization. Next, the thus metalized reduced iron 25 is cooled by a cooler 28 and, after it develops mechanical strength capable of withstanding a handling operation when and after it is discharged, it is discharged to the outside of the furnace by a discharge device 22. Just after discharge of the metalized reduced iron 25, a new dry pellet or a dry briquette (starting material 24 of reduced iron) is inserted; and, the above process is repeated to thereby produce reduced iron (see, for example Patent Document 1).
In the rotary-hearth furnace used for the production of such reduced iron, an exhaust gas generated in the furnace is guided from an exhaust gas discharge area placed on the circumference of the rotary-hearth furnace to an exhaust duct connected to the ceiling part of this exhaust gas discharge area. The exhaust gas guided to the exhaust duct is treated by exhaust gas treatment equipment placed in the intermediate part or downstream of the exhaust duct, and is then discharged to the outside of this system. However, there is known a problem that, as various volatile impurities are generated during the reducing process or melting process of the reduced iron material, the exhaust duct can clog or corrode, or a refractory can be damaged.
Thus, as a method for operating such conventional exhaust gas treatment apparatus, there is proposed a method for preventing the clogging of an exhaust gas suction duct or the damage of a lined refractory (see Patent Document 2). In this method, by supplying any one or more of an inert gas, gas-water state water and air to an exhaust gas of 1100° C. or higher discharged from the rotary-hearth furnace, the exhaust gas temperature within the exhaust gas suction duct is cooled to from 900 to 1100° C.
Further, recently, there has been developed a process for producing a high-purity granular metallic iron. In this process, a starting material including a carbonaceous reducing material and an iron oxide-containing material is heated in a reducing melting furnace such as a rotary-hearth furnace to solid-reduce the iron oxide in this start material, and the yielded metallic iron is then further heated to be molten, and it is aggregated while separating it from the slab components.
However, this process for producing the granular metallic iron has a problem regarding the increase of the amount of the exhaust gas and increases of the exhaust gas temperature. That is, when the amount of the exhaust gas increases, since the capacity of apparatus equipped downstream thereof such as the exhaust gas duct apparatus and exhaust gas treatment apparatus is increased, the facility cost increases as well as the running cost necessary to solve the problem involved with dust adhesion or accumulation increases. Also, when the exhaust gas temperature increases, higher heat resistance is required in the equipment placed downstream, which further increases the facility cost and running cost.